The iconic red buses plying London's roads no longer accept cash as payment for fare.

Beginning July 6, the buses will only take-on passengers that pay with "Oyster Cards," a blue credit-card-sized contactless smartcard that can be "topped-up" to extend its stored-value. When boarding a London bus, a user must touch his Oyster card only once at a special terminal inside the bus since buses have a single flat fare.

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This card is also used by passengers on the London Underground, the Docklands Light Railway, the London Overground and on trams, among others.

The move to adopt the Oyster card to pay bus fare has been controversial since public opinion polls have consistently shown strong public opposition to the sole use of the card.

The Oyster Card, first adopted on other forms of public transport in 2003, is the brainchild of the Transport for London (TfL), a local government body that runs much of the transport system in Greater London.

In answer to critics of the Oyster Card's use on buses, TfL said only one per cent of bus passengers now pay their fare in cash. It claims the mandatory use of the Oyster Card will result in London saving US$223 million by 2023.

TfL, however, has not quieted fears that thousands of commuters without Oyster Cards will be left stranded each day under its cash-free system.

To address this issue, TfL has introduced the "One More Journey" initiative that allows commuters to make a single journey if they do not have sufficient credit on their Oyster card.

Critics said this palliative doesn't solve the common and widespread problem of commuters that either do not having a card at all or have found out that their Oyster Cards don't work.

Then there's the problem of stolen Oyster Cards. TfL data showed that 2,115 Oyster Cards were lost, stolen or malfunctioned on average every day, for a total of 770,000 per month in 2013.

The Green Party said that without an Oyster Card, passengers won't be able to pay cash as an alternative and will therefore have to appeal to a bus driver's sympathy. This situation could be especially dangerous for more vulnerable segments of the population such as the elderly and the infirm.

TagsOyster card, London bus